One area of considerable concern in dealing with fixed bed catalytic reactors concerns the manner in which the liquid introduced to the reactors is distributed with respect to the fixed bed contained therein. There are a number of different types of distributors presently in use for facilitating the uniform application of liquid over the upper face of a catalyst bed.
One common type of system such as described in U.S. Pat. No. 4,126,539 involves an inlet deflector cone cooperating with a splash plate and liquid distributor trough to pass liquid into the reactor to two distributor trays which are designed to facilitate the uniform spreading of liquid over the upper face of a catalyst bed. The distributor trays contain a series of spaced risers which have dual functions. They permit vapor to pass the tray and also serve as liquid conduits due to weir slots cut into the sides of the risers.
There are several difficulties encountered in this basic design. The liquid is carried past the tray by the risers and thus, the number of points at which the liquid is introduced to the upper face of the bed is limited by the number of risers which can be uniformly positioned on the tray. This limitation is aggravated by the fact that the risers are of relatively large diameter and it is necessary to fabricate the tray in sections. Accordingly, as the number of liquid introduction points is decreased, the depth to which the liquid must penetrate the catalyst bed to reach equilibrium distribution increases, and catalyst utilization in the upper bed is thereby impaired. Additionally, due to the nature of liquid flow through weirs, the uniformity of liquid distribution effected by this type of design is very sensitive to tray unevenness introduced during fabrication or installation.
Another example of a known distributor is the mixed-phase flow distributor for packed beds of U.S. Pat. No. 3,524,731. That distributor was designed primarily to accommodate wide variations in liquid feed rate. Liquid flow is normally through liquid downpipes. At very high liquid rates some liquid overflows into the vapor chimneys through triangular weirs. However, during normal operation the chimneys do not carry liquid and hence do not contribute to the number of liquid streams entering the bed. Also, during periods when they carry liquid there would be a great variation in the liquid flow through the chimneys compared with that through the tubes.
U.S. Pat. No. 3,353,924 shows a somewhat different approach, however, flow into the liquid tubes is still through a pair of notched weirs and the disadvantages mentioned above are applicable with respect to this system as well. There is no liquid flow in the vapor chimneys, and the number of uniformly spaced liquid streams which can be placed on the tray is therefore limited.
Attention is again directed to U.S. Pat. No. 4,126,539 which is the system initially discussed above. Liquid flow is through the vapor downcomers only, by a combination of hole and weir flow. There is no attempt or suggestion in the system of the patent to utilize the tray area between the downcomers for liquid distribution, and the use of weir flow makes the distribution pattern vulnerable to variations in tray level.
Other approaches appear in U.S. Pat. Nos. 4,126,540 and 4,140,625 where liquid flow is through holes in downcomers only. There is no attempt to make use of the tray area between downcomers and the size of the downcomers coupled with the need to maintain tray mechanical integrity prevents maximization of the number of liquid streams entering the catalyst bed.
Liquid distribution is also of concern in other environments. For example in U.S. Pat. No. 2,924,441 the disclosure is related to the design of a liquid distributor for gas/liquid contact such as in gas absorption or distillation in a packed tower. Emphasis is on mechanical construction of such a nature as to permit application of a corrosion resistant coating such as glass or Teflon. The distributor described makes no attempt to address the special need for good initial liquid distribution found in cocurrent downflow catalytic reactors and therefore would not lead one to the realm of the present invention.
Another patent of interest in the general state of the art is U.S. Pat. No. 2,772,780 which, although concerned with good liquid distribution, deals with the contact of upward flowing liquid with downward flowing solids. Since there are no vapor chimneys occupying part of the tray area, uniform layout of liquid streams is not a problem. Thus, the teaching would not lead one to a concept such as that of the present invention.